Republican presidential candidate Mitt Romney plans to name Congressman Paul Ryan as his vice presidential running mate on Saturday in a move that will bring the debate over how to reduce government spending and debt to the forefront of the race for the White House.

A Republican official said Romney would introduce Ryan at the retired battleship USS Wisconsin - coincidentally named for Ryan's home state - in Norfolk at about 9 a.m. EDT (1300 GMT).

The announcement marks the end of a months-long search for a running mate to challenge Democratic President Barack Obama and Vice President Joe Biden in the Nov. 6 election, and will serve to energize conservatives who staged a big effort in recent days to persuade Romney to pick Ryan.

It comes as Romney starts a bus tour on Saturday through four battleground states he needs to win in November - Virginia, North Carolina, Florida and Ohio.

Romney's move is a bold one as he finds himself suddenly falling behind slightly in what has been a neck-and-neck race with Obama, according to recent polls, in a campaign that is based largely on the weak U.S. economy.

The selection of Ryan, 42, brings a measure of youthful exuberance and energy to the Republican ticket as party activists prepare to gather in Tampa late this month for the convention that will formally nominate Romney as the presidential nominee. Romney aides hope the vice presidential choice and the enthusiasm produced by the convention will give him a bounce in the polls.

Conservatives had been urging Romney to pass over tried-and-true Republicans like Ohio Senator Rob Portman and former Minnesota Governor Tim Pawlenty, two other big names on his short list, and instead go with Ryan.

But many Republicans had wanted Romney to play it safe and go with either the more experienced Pawlenty or Portman. A Republican source said Portman got a call from Romney saying he had not received the running mate nod, and other reports said Pawlenty got a similar call.

Ryan's selection immediately draws attention to a budget plan which Ryan proposed as House budget chairman that would include unpopular cuts in government health programs for the elderly and poor. Democrats are eager to pounce on that issue, particularly in Florida, where many seniors live and which could be a critical state in the November election.

Romney's decision to pick Ryan showed he is comfortable with having this debate over the future role of government and ways to rein in out-of-control spending and debt. He has endorsed parts of Ryan's budget.

"Conservatives are going to be very energized because this is a demonstration that Romney was willing to make a bold pick," said Republican strategist Matt Mackowiak. "It may not be what he wanted to do three or six months ago, but I think this is as significant a choice as he could have made."

But Bill Burton, senior strategist at the pro-Obama group Priorities USA, suggested Ryan could prove a liability.

"If it's really Ryan, Romney will have picked one of the only people who could have had an impact in the race. But not the way he wants," he said in a tweet.

The conservative Weekly Standard magazine reported that the Romney campaign had begun to prepare a vigorous effort to support Ryan as the vice presidential pick - "something now likely to happen soon".

Often likening Ryan to Ronald Reagan, conservatives say the Wisconsin lawmaker's supposed drawbacks as a candidate - mostly stemming from the steep cuts he has proposed in social safety net programs - are actually strengths that could bring heft, content and perhaps a spark to Romney's campaign.

Romney bonded with Ryan during the Wisconsin Republican primary battle last spring when Ryan campaigned enthusiastically for the former Massachusetts governor and they often appeared together.

For Romney, an outsider to Washington, Ryan would provide some expertise in dealing with Congress. But as a long-time member of the House, he lacks executive experience.

The challenge for Romney will be to introduce Ryan to the broader American electorate. While well known in Washington and among conservatives, most voters are not familiar with him.